1. Field of the Invention
The present invention relates to a method for manufacturing a ceramic heater and more particularly to a method for manufacturing a glow plug employed for starting a diesel engine and a glow plug.
2. Description of the Related Art
Conventionally, a ceramic heater of the type used for a glow plug employed for starting a diesel engine is manufactured in the manner described below. FIG. 1 illustrates a process for producing a ceramic heater from a material powder. First, a conductive ceramic powder 3, an insulating ceramic powder 5, a sintering aid powder 7—all these powders being finely pulverized in advance—and a solvent 9 are mixed by use of an attritor, a stirring pot 16, or the like, thereby obtaining a slurry 10 (1-1). The slurry 10 is placed in shallow containers 12 or the like. The shallow containers 12 are arranged within a stationary drying apparatus 14, and hot gas HG is circulated within the stationary drying apparatus 14 (1-2). The solvent 9 is thus evaporated, thereby yielding dry cakes 18 (symbol OG denotes outflow gas). The dry cakes 18, together with a medium 22 (pebbles), are placed in a ball mill 20 and crushed (1-3), thereby yielding a heating-element material powder 24. The heating-element material powder 24 and a binder 26 are kneaded and formed into a green resistance-heating element 28 by an injection molding process (1-4). The green resistance-heating element 28 is accommodated within a green ceramic substrate 30. The resultant assembly is fired through a method such as HIP, thereby yielding a ceramic heater 1 (1-5 and 1-6). Other components such as a metallic shell 32 and a metallic terminal 34 are assembled into the ceramic heater 1, to thereby fabricate a ceramic glow plug 36 (1-7).
As mentioned above, conventionally, the slurry 10 to be dried by use of the stationary drying apparatus 14 contains as the solvent 9 an organic solvent such as an alcohol, hexane, or xylene.
Recently, the influence of chemical substances on the environment has been discussed extensively. Under such circumstances, a tendency to limit use of organic solvents has arisen. The ceramic heater manufacturing field is no exception to this. Development of a process for obtaining a heating-element material powder without use of an organic solvent is urgently demanded.
Generally, water is used as a solvent in preparing a slurry in which insulating ceramic powder serves as a sole powder ingredient. However, the present inventors have found a problem involved in the use of water as a solvent. Specifically, the present inventors prepared a slurry by use of water in place of an organic solvent and, from the slurry, manufactured ceramic heaters for use in a glow plug, through the aforementioned conventional method. The ceramic heaters were subjected to a repetitive-electricity-application durability test in which the heaters were repeatedly subjected to a cycle consisting of electricity-effected heating and standing to cool. A large number of the tested ceramic heaters were found to be of low durability; i.e., a disconnection fault occurred after a small number of test cycles. Such ceramic heaters cannot be used in a glow plug, which must endure tens of thousands of electricity application cycles.